Cellular telephone users (cellular telephone users and cellular telephones are also referred to herein as “terminals”) in the downlink (base station to terminal) path of a cellular system who are on or near the boundaries between cells or sectors usually have low signal-to-interference ratio (SINR) because of the strong interference from neighboring cells or sectors. These terminals often have poor connectivity with their respective base stations as a result.
FIG. 1 is a block diagram of a prior art cellular telephone system. FIG. 1 includes three cells, labeled A, B, C, in a typical arrangement. Each cell includes three respective sectors labeled 1, 2, and 3. The cell and sector boundaries are indicated by lines. Each sector typically serves its own set of terminals.
Typically the edge of a cell represents a boundary where the signal strength from a serving base station A (not shown) is comparable to the strength from one or more neighboring base stations (base station B, C, and so on, not shown). The transmissions from base station B, in contact with its own terminals, may then appear as interference to the terminal on the boundary, thus causing low SINR and impairing the data rate to the terminal.
In a similar fashion, the edge of a sector represents a boundary where the transmission beam-pattern allows transmissions from one sector to spill over into another sector as interference. Thus, a terminal being served in sector 1 may also see interfering transmissions from sectors 2 and 3. In an orthogonal-frequency-division multiple-access (OFDMA) system, time and frequency are usually divided into sub-units called symbols (in time t) and subcarriers (in frequency k). OFDMA systems are also referred to as orthogonal-frequency-duplex multiple-access systems. The base station typically assigns multiple time symbols and subcarriers to carry the data from the base station to the terminals. Each terminal within a sector is usually assigned a distinct subset of the available symbols and subcarriers. Such subsets of symbols and subcarriers are denoted “slots” herein. The number of subcarriers, the number of symbols, and the level of modulation (which is usually a function of the SINR) determines the data rate to the terminal.
In a “frequency re-use one” system, every cell and sector is free to utilize all of the subcarriers and symbols without regard for the subcarriers and symbols used in other cells or sectors. Such a system therefore has significant interference between sectors and cells, especially on their boundaries. In systems with higher frequency re-use, the interference tends to be less on sector and cell boundaries but can appear elsewhere.
One way of expressing the effect of interference in cellular communication systems is provided by Equations 1-3 below. In this example, Base station A transmits on subcarrier k and time t on Sector 1 the signalsk,t(1)=uk,t(1)  Equation 1where uk,t(1) is a unit-energy data-symbol intended for a terminal in Sector 1. Base station A transmits on the same subcarrier k and time t on Sector 2 the signalsk,t(2)=uk,t(2)  Equation 2where uk,t(2) is a data-symbol intended for a distinct terminal in Sector 2. The terminal in Sector 1 at time t and subcarrier k receivesrk,t=hk,tuk,t(1)+gk,tuk,t(2)+wk,t  Equation 3
where the channel between the terminal and its antenna (in Sector 1) is hk,t and gk,t. The terminal might have trouble decoding its intended message uk,t(1) if the channel magnitudes |gk,t| and |hk,t| are comparable.